CMH05MT900 Common Mode Choke 90 Ohm Specs and Design Application
CMH05MT900The CMH05MT900 is a 90 Ohm common mode choke rated for 400 mA continuous current, designed specifically for signal line filtering in space-constrained surface-mount assemblies. This component from Viking Tech addresses the common-mode noise suppression requirements of modern differential signal interfaces operating at switching frequencies up to 100 MHz.
In industrial Ethernet physical layer (PHY) implementations, particularly 100BASE-TX and 100BASE-T1 automotive variants, the signal integrity challenge centers on common-mode noise generated by gigabit-speed switching transients in the power stage and nearby DC-DC converters. The IEEE 802.3 specification mandates common-mode rejection of at least 30 dB at the critical 100 MHz Nyquist frequency. Without adequate filtering, conducted emissions from the Ethernet interface can cause adjacent receiver desense, bit-error-rate degradation, and failure of CISPR 25 Class 5 radiated emissions limits common in automotive ECU designs.
Quantified Component Requirements for 100BASE-TX Signal Line Filtering
An effective common-mode choke for this application must meet five quantified criteria:
- Common-mode impedance: 60-120 Ohm at 100 MHz to suppress differential-to-common mode conversion noise without distorting the differential signal
- DC current rating: At least 300 mA to accommodate the IEEE 802.3 100BASE-TX transmit current of 200 mA plus 50% design margin for startup transients and aging
- DC resistance maximum: Below 500 mOhm to limit voltage drop across the choke to less than 50 mV, preserving receiver 1V peak-to-peak signal swing tolerance
- Package footprint: 0805 or smaller to fit alongside magnetics modules in 2-layer PCB layouts typical of cost-optimized industrial designs
- Operating temperature range: -40°C to +85°C to cover enclosure-rated equipment operating in uncontrolled factory environments
Why the CMH05MT900 Fits These Requirements
The Common Mode Chokes family from Viking Tech provides a 90 Ohm impedance at the critical 100 MHz frequency, positioned in the middle of the 60-120 Ohm design window. The 2.00 mm × 1.20 mm 0805 footprint enables placement directly adjacent to the RJ45 connector or automotive header, minimizing stub lengths that would otherwise create parasitic resonances above 200 MHz.
| Parameter | Value | Engineering Meaning |
|---|---|---|
| Impedance @ Frequency | 90 Ohm @ 100 MHz | Targets common-mode noise at the Ethernet fundamental clock frequency; values between 60-120 Ohm are typical for 100BASE-TX |
| Current Rating (Max) | 400 mA | Provides 2× margin over the 200 mA nominal transmit current; ensures no saturation during inrush or fault conditions |
| DC Resistance (Max) | 350 mOhm | Limits DC voltage drop to 140 mV at rated current; well below the 500 mOhm maximum acceptable for 3.3V logic margin |
| Voltage Rating - DC | 50 V | Exceeds the 3.3V PHY supply by 15×, providing dielectric safety margin for ESD events up to ±8 kV contact |
| Operating Temperature | -40°C ~ 85°C | Covers industrial enclosure temperature classes; derating required above 85°C — consult datasheet for current reduction factor |
| Package / Case | Horizontal, 4 PC Pad 0805 | Compact footprint for dense 2-layer automotive ECU layouts; requires standard pick-and-place with 0.5 mm pad geometry |
| Filter Type | Signal Line | Optimized for differential pair common-mode filtration, not power-line bulk current handling |
| Features | ESD | Integrated ESD structure provides ±8 kV contact discharge protection; reduces external TVS diode count |
The 90 Ohm impedance at 100 MHz is the most critical spec for this application. Common-mode chokes with impedance below 60 Ohm may not provide sufficient common-mode rejection to meet CISPR 25 Class 5 limits, while impedance above 120 Ohm can begin to degrade the differential return loss of the Ethernet channel. The CMH05MT900 lands in this Goldilocks zone, enabling 30-35 dB common-mode attenuation at the frequency of highest noise energy from the PHY's 125 MHz transmit clock.
The 400 mA current rating with 350 mOhm DCR also provides practical design margin. A 100BASE-TX transmit pair draws 180-220 mA typical; the 1.8× margin on current ensures the ferrite core does not reach saturation even during the ±1A transient when the Ethernet transformer common-mode center tap is momentarily shorted during hot-plug events. The 350 mOhm resistance translates to a 70-77 mV DC drop at nominal current, which remains within the 200 mV tolerance acceptable for the internal PHY bias voltage.
Typical Circuit Topology and Signal Flow
In a standard 100BASE-TX PHY-to-connector interface, the signal flow follows: PHY transmit differential output (TX+ / TX-) → series AC coupling capacitors (0.1 μF) → CMH05MT900 common-mode choke → Ethernet transformer (1:1 ratio, 350 μH) → RJ45 connector with integrated magnetics. The common-mode choke sits on the PHY side of the transformer, filtering noise that originates from the PHY's switching current draw before it couples through the transformer capacitance.
The four-terminal package layout places each differential pair leg on opposite sides of the 0805 body: pin 1 (TX+ IN), pin 2 (TX- IN), pin 3 (TX+ OUT), pin 4 (TX- OUT). The PCB trace routing must maintain 100 Ohm differential impedance from the PHY pins through the choke pads to the transformer. For a 2-layer board with 1 oz copper, this typically requires 8 mil trace width with 8 mil spacing over a 4 mil FR4 dielectric.
For the receive path, a second CMH05MT900 is placed symmetrically between the transformer center-tapped secondary and the PHY receiver inputs. The ESD feature provides a low-impedance path to ground for ±8 kV contact discharges that may couple through the transformer inter-winding capacitance during hot-plug or unshielded cable events.
Design Considerations: Thermal, Life, Derating, EMC
Operating at 400 mA current through 350 mOhm DCR produces 56 mW of self-heating per choke. In an enclosure with ambient temperature of 70°C and no forced airflow, the internal temperature rise is approximately 15-20°C. This keeps the choke well below the 85°C absolute maximum, but must be considered when multiple chokes are placed in close proximity (e.g., four chokes for a 4-port switch). The aggregate 224 mW heating can raise local board temperature by 8-12°C, requiring a 10% current derating to avoid accelerated aging of the ferrite.
For CMH05MT900 datasheet derivation at elevated temperatures, the standard approach is to use the 85°C rating as the baseline: reduce current by 15% for every 10°C above 85°C ambient. Since the part is not rated above 85°C, designs targeting 105°C enclosure environments (typical of automotive under-hood ECUs) must select a higher-temperature grade choke such as the sibling CMH06MT222 which extends to 105°C.
EMC coupling paths through the common-mode choke must be considered in board layout. The choke's ferrite core can act as an unintentional antenna if traces carrying high-frequency switching noise (e.g., the PHY's 125 MHz clock oscillator output) are routed within 2 mm of the choke body. Keep all digital clock lines at least 5 mm away from the choke placement, and ensure the ground plane under the choke is continuous with stitching vias to the return path.
Common Application-Specific Issues and Solutions
Issue: EMI fails CISPR 25 Class 5 at 125 MHz. The third harmonic of the 25 MHz data clock couples through the common-mode choke's parasitic capacitance. Solution: Add a 10 pF capacitor from each differential line to ground after the choke to create a 60 MHz LC low-pass filter, attenuating harmonics above the choke's self-resonant frequency.
Issue: Bit error rate increases during ESD testing. The integrated ESD structure may clamp at a voltage that violates the PHY's common-mode input range (typically ±1 V for 100BASE-TX). Solution: Verify that the PHY's ESD tolerance matches the choke's clamping voltage (consult CMH05MT900 datasheet for clamping characterization), and add serial 10 Ohm resistors between choke output and PHY pins to limit clamp current.
Issue: Differential return loss fails IEEE 802.3 return loss mask. The choke's internal winding capacitance creates a resonant dip at 250-300 MHz. Solution: Use the CMH05MT900's 90 Ohm impedance to intentionally cancel a portion of the transformer's 350 μH leakage inductance. Tune the trace length between choke and transformer to achieve a 180° phase shift at the resonance frequency.
Frequently Asked Questions About CMH05MT900
What is the CMH05MT900 equivalent or cross reference part?
The CMH05MT900 can be cross-referenced against sibling parts from Viking Tech including CMH06MT222 (larger 1206 package with 600 Ohm impedance), CBM02YTYH121, and CB05YTYH331. For direct equivalent comparison, the 90 Ohm @ 100 MHz 400 mA rating fits applications that would otherwise use parts from manufacturers like Murata's DLW21SN90SQ or TDK's ACM2012-900-2P. Always verify the pinout orientation (horizontal 4-pad) and ESD rating when substituting.
Where can I find the CMH05MT900 datasheet?
The official CMH05MT900 datasheet is available from Viking Tech's product documentation portal and through authorized distributor seekcomp. The datasheet provides impedance vs frequency curves from 1 MHz to 1 GHz, temperature derating curves for current above 85°C, ESD clamping voltage characterization, and recommended PCB land pattern dimensions including solder paste stencil apertures for reflow soldering.
What is the CMH05MT900 application circuit for Ethernet?
A typical CMH05MT900 application circuit places the choke between the PHY's differential output (TX+/TX-) and the Ethernet transformer. The input side connects to 0.1 μF AC coupling capacitors, while the output side connects to the transformer center-tapped primary. For the receive path, a second choke is placed between the transformer secondary and the PHY receiver pins. Placement must maintain 100 Ohm differential impedance and keep all clock traces at least 5 mm from the choke body to avoid EMI coupling.
Is the CMH05MT900 RoHS compliant?
Yes, the CMH05MT900 is RoHS compliant. The part's construction uses lead-free terminations with nickel barrier and tin plating. The ferrite core material is free of restricted substances as defined by EU Directive 2011/65/EU. For environmental compliance certificate and REACH SVHC statement, consult the product page on seekcomp or request documentation from Viking Tech.
Design engineers should verify the CMH05MT900's self-resonant frequency against their specific operating conditions by measuring the S-parameters with a VNA sweep from 1 MHz to 1 GHz. For 100BASE-TX designs, the 90 Ohm impedance at 100 MHz provides the needed common-mode rejection margin, while the 400 mA rating ensures robust operation across the industrial temperature range. Keep the choke placement within 15 mm of the connector and maintain continuous ground reference beneath the part for optimal ESD and EMI performance.
